High silica zeolites are extensively used as catalysts, sorbents, ion-exchangers, separation media and pollution control agents in the petroleum, chemical and process industries. Although some commercially important zeolites, such as ZSM-5 and beta, can be directly synthesized in high silica forms (i.e. Si/Al&gt;10), others, such as Linde-A and faujasite materials, must be prepared by first making a lower Si/Al ratio version then dealuminating them to form the high silica analog. Many such dealumination methods are known in the art and are generally referred to as "secondary synthesis". The term "secondary synthesis" refers to the process of changing the zeolite framework composition by chemically reacting various cation exchanged forms of the "as synthesized" zeolite (or mineral form) with a variety of reagents. An early dealumination process taught acid treatments of the mineral clinoptilolite (Barrer and Makki, Canad.J. Chem., 42, p.1481, (1964)), the process had its major impact with the discovery that steam treatments of Na, NH.sub.4 -Y produced enhanced stability materials (U.S. Pat. No. 3,293,192). The latter, called "ultra-stable Y" (US-Y), have been a major research subject since the late 1960's. These processes, and their applications, have recently been reviewed by Scherzer in several publications (e.g. Amer. Chem. Soc. Symp. Ser. 248, p. 157-200 (1984): Catal. Rev. Sci. Engrg., 31(3), p.215-354 (1989)). Examples of such processes include steaming, as described in U.S. Pat. No. 3,293,192; acid leaching, as described in U.S. Pat. No. 3,506,400; the use of complexing reagents (e.g. EDTA), as described in U.S. Pat. No. 4,093,560; the use of SiCl.sub.4 vapor, as described in Beyer and Balenkaya, (in "Catalysis by Zeolites", Stud. Surf. Sc. Catal. v.5, p. 203, (1980), Elsevier)); and the use of SiCl.sub.4 in non-aqueous solvents, as described in WO 88/01254; and the use of CHF.sub.3 as described in U.S. Pat. No. 4,275,046. The resulting products are often called `ultra stable` faujasites because of their enhanced thermal and hydrothermal stability. Finally, another technique for increasing the Si:Al ratio is the use of an aqueous solution of (NH.sub.4).sub.2 SiF.sub.6, as described in U.S. Pat. No. 4,503,023. All of the above are non-limiting examples of conventional methods of "secondary synthesis" for the dealumination of zeolite materials.
Such methods, used in a multiple repetitive mode, have been used to make a near pure silica form of faujasite (Scherzer, J. Catal., vol. 54, p. 285 (1978)). However, there are significant differences in the catalytic properties of ostensibly similar materials. Clearly different routes to the same composition produce microscopically different products as demonstrated in many publications (e.g. Dwyer et al, Chem.Commun., p.422 (1981); Stud. Surf. Sci. Catalysis, vol. 69, pp. 1-24, (1991)), whether they are obtained by progressive de-alumination, or re-alumination of de-aluminated materials. Variations in secondary mesopores (both in pore volume and pore diameter distributions) and Al distributions (speciation and heterogeneous distribution within or on the zeolite crystals) are difficult to control in all of these processes. Generally, the more processing needed to achieve a target composition the lower its yield (i.e. material is dissolved away) of the crystalline product and the greater the amount of its amorphous component in the product.
We recognise that there may be some catalytic advantages to a degraded zeolite material containing large mesopores (&gt;10 nm), particularly in the conversion of wide composition range "heavy" or "resid" feedstocks containing large multi-ring aromatic molecules and high heteroatom (S, N) contents, as converted in fluid catalytic cracking. However, the products of the present invention are targeted at more selective processing and have more controlled and selective properties, have low yield losses, require less processing to achieve target properties and therefore produce fewer waste products and polluting effluents.
Therefore, there is a need in the art for processes for producing faujasitic zeolitic materials in high yield, having relatively high Si/Al ratios, as well as the resulting high silica zeolite materials themselves.